Understanding the Monterey Formation and Similar Biosiliceous Units across Space and Time
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The Monterey Formation is a Miocene marine unit that occurs extensively in the Coast Ranges and in the continental margins of California, and analogous biosiliceous deposits are found around the Pacific Rim and elsewhere in the world. Classic studies on the diatomaceous deposits that characterize the hemipelagic/pelagic facies of the Monterey Formation have been key to understanding the oceanographic and tectonic conditions that lead to the preservation of large volumes of organic-rich hemipelagic biosiliceous sediments, and the properties of these sedimentary deposits once they convert into rocks. This volume presents a collection of recent studies on the Monterey and other similar biosiliceous deposits that offer modern and updated interpretations of this classic unit and its analogues. The volume is dedicated to the memory of Professor Bob Garrison.
Nanometer-scale pore structure and the Monterey Formation: A new tool to investigate silica diagenesis
Published:September 26, 2022
Cynthia M. Ross, Anthony R. Kovscek, 2022. "Nanometer-scale pore structure and the Monterey Formation: A new tool to investigate silica diagenesis", Understanding the Monterey Formation and Similar Biosiliceous Units across Space and Time, Ivano W. Aiello, John A. Barron, A. Christina Ravelo
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The Monterey Formation and related formations in California have long been the subject of field and laboratory studies on silica diagenesis. Biogenic or amorphous silica (opal-A) alters to a more-ordered opal-CT and eventually to the crystalline end member, quartz, with increasing burial depth and temperature. Low-pressure nitrogen sorption serves as an indicator of silica alteration by detecting the nanometer-scale pore structures associated with opal-CT while excluding contributions from larger pores. To apply this method, calibrations with known compositions are not required, sample preparation and measurements are straightforward, hazardous waste is not generated (as with mercury porosimetry), and subtle changes in silica phase are readily detected.
Nitrogen desorption isotherms, collected on mini cores (~0.8 cm diameter × 1 cm) after outgassing at 50 °C and processed using the Barrett-Joyner-Halenda method, provide nanometer-scale pore throat size distributions (nPSD), pore volumes (nPV), and surface areas (nSA). A scatter plot of nPV and nSA reveals two distinct trends. Samples with more nSA per unit volume contain opal-CT, either in transition from opal-A or completely converted. The other nSA trend consists of opal-A and quartz samples in the small nSA and nPV range, whereas samples with small nSA and large nPV also contain opal-CT and are in transition to quartz. These distinct trends are also apparent in the nPSD. Samples with more nSA exhibit a peak between 4 and 10 nm, whereas samples with less nSA have a broad peak between 10 and 100 nm if they contain opal-CT. Images collected via scanning electron microscopy reveal that opal-CT morphologies account for these differences.
- biogenic processes
- chemical composition
- electron microscopy data
- framework silicates
- Monterey Formation
- SEM data
- silica minerals
- size distribution
- United States
- X-ray diffraction data
- Barrett-Joyner-Halenda method